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| Book/Report | FZJ-2018-01074 |
; ;
1977
Kernforschungsanlage Jülich, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/16999
Report No.: Juel-1448
Abstract: Prehandling and discharge cleaning methods applied in toroidal magnetic systems with metallic walls have been surveyed and their efficiency in depressing the initial concentration of low Z contaminants (mainly oxygen, but also carbon) have been compared. Chemical reactions between atomic (H° and H$^{+}$ ) hydrogen species and chemically bound oxygen and lattice-bound carbon in the surface-near layers are found to play a dominant role in the impurity release. Discharge cleaning in Tokamaks tends to crack and ionize the resulting water and hydrocarbons, leading to impurity ions which recombine chemically with the walls at the end of the discharge cycle. An ultrahigh vacuum apparatus has been built, in which the cleaning efficiency of H° and H$^{+}$ is investigated. The flux densities of the atomic species have been varied between 10$^{14}$ and 5 x 10$^{16}$ atoms/cm$^{2}$ s, covering the range of the averaged values of present day confinementdevices and future reactors. The cleaning efficiency is found to be high. A chemical kinetics model describes the mechanisms involved. An H$_{2}$ blistering-like effect is observed and is attributed to the H + H$^{2}$ recombination underneath the surface. This process influences the recycling phenomena and probably also accounts for the early appearance of metallic impurities in tokamak experiments. Conclusions to be shown from our measurements concerning phenomena observed in confinement systems are presented.
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